This grant is submitted in response to the NIMH Neural Systems Program announcement number 92-07 in order to develop a model with which to explore the relationships among the various sensory cortices, the basal ganglia, and the midbrain and their interactive role in attentive and orientation behaviors. The influence of cortex over midbrain behaviors has long served as a paradigm for understanding the manner in which 'higher' structures regulate subcortical sensorimotor behaviors. This relationship is underscored by observations that cortical and subcortical lesions often produce similar behavioral dysfunctions, presumably because they disrupt different portions of a common integrated system. However, the actual neuronal circuitry by which the cortex controls these behaviors via its influence on the neural activity of the midbrain (i.e., superior colliculus) is poorly understood. Currently, cortical influences on superior colliculus neurons have been overwhelmingly interpreted as being mediated solely via direct connections, an assumption reflecting the robust nature of direct corticotectal projections which have been detailed repeatedly. However, we believe that this issue requires serious reevaluation. It is likely that there is an indirect corticotectal pathway via the basal ganglia that will prove to be an important linkage between cortex and the superior colliculus. While there has long been theoretical support for this indirect pathway, as well as for the functional linkage between the basal ganglia and the superior colliculus, there has been little experimental effort devoted to establishing the relationship among these structures. New preliminary anatomical and physiological evidence is consistent with an indirect linkage. A new systems model is proposed for understanding cortical-superior colliculus relationships that is based on the premise that all sensory cortices exert their influences on superior colliculus neurons via direct and indirect pathways. The principal aim during this grant period will be to determine whether this multimodal systems model can be supported by anatomical and physiological evidence. These relationships are likely to have a direct impact on our understanding of how diseases of the basal ganglia disrupt attentive behavior, which is intimately related with the superior colliculus.